Conventional image sensor packages leave much room for improvement at the wafer level. For example, conventional backside illuminated (BSI) CMOS image sensor packages waste pixels by design. There is also an undesirable leakage current between color pixels. The conventional designs impose limitations on color reproduction, image detail, and speed/form factor. These limitations are variously caused by differences in the way that different wavelengths of light (different colors) interact with conventional monolithic filters and uniform image sensor materials. Blue wavelengths penetrate in a relatively shallow manner while red wavelengths penetrate much deeper.
As shown in FIG. 1, a conventional unbalanced Bayer filter mosaic array 50 is a color filter array (CFA) for arranging red-green-blue (RGB) color filters on a square grid of photosensors. The filter pattern is 50% green, 25% red and 25% blue, providing a RGGB filter 60. The conventional unbalanced Bayer filter mosaic array 50 has twice as many green filters as blue or red, which decreases accurate color reproduction of individual pixels. Due to the transmission spectral profiles 70 of the dyes commonly used in Bayer filters, the quantum efficiency of the red filters is significantly greater than that of the green and blue filters, which are close to each other in overall efficiency. After an image stream passes through the conventional Bayer filter 50, the materials of the conventional sensing surface are monolithic, so the thickness and doping profile of the sensor materials cannot be optimized for each color. Compensating for these conventional drawbacks results in additional complexity in conventional CMOS BSI designs for image sensor packages. Moreover, state-of-the-art high-definition (HD) video of 16M or greater calls for an approximately 1.0 nanosecond sensing and processing time.
In the conventional designs 50, three of the four pixels (the RGG pixels of a RGGB group of pixels) pass yellow light (at wavelength 585 nm) plus 20% blue light (435 nm). A blue pixel (B) passes nothing but blue light (435 nm). This imbalance causes a reduction in spatial resolution and sensitivity. Conventional designs compare the color (chromaticity) and intensity (luminosity) of a given pixel with neighboring pixels to reconstruct the signal to R, B, G, using various linear, next-neighbor, cubic, cubic spiral, cubic spline, and sinc interpolator patterns. For edge pixels with no neighboring pixels, the reconstruction is wasted. Other inherent problems with conventional designs include noise and crosstalk, with on-chip suppression and active decoupling executed at the sacrifice of speed.